In HTML 4.01, you can use any metadata names, so using abstract would be allowed. But it should be defined in a profile:

This specification does not define a set of legal meta data properties. The meaning of a property and the set of legal values for that property should be defined in a reference lexicon called a profile.

Metadata can only be useful if it’s use/scope is defined (e.g. in specifications) resp. documented (e.g. by metadata consumers).

Another answer could be which meta tags are recognised by search engines. For example, abstract is not understood by Google as a valid meta tag whereas description is. Other search engines may have different criteria.